问题补充：
The boys followed the marks that they , until finally they found their way back to the campsite.
A. left
B. have left
C. had left
D. have been left
答案：
A. left
B. have left
(答案→)C. had left
D. have been left
解析：考查时态：句意：男孩跟随着他们留下的记号，直到最后他们到了回到营地的路。Leave the marks是发生在follow the marks之前的，所以用过去完成时，选C。
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